Contact lenses which derive their oxygen permeability from organosiloxane moieties require a substantial content of organosiloxane to provide sufficient oxygen transport to the cornea.
In general organosiloxane molecules tend to be incompatible in many compositions which include other monomers. For example, dissolving organosiloxane in methyl methacrylate and polymerizing said solution often results in an opaque material unsuitable for contact lens use.
Prior art has demonstrated that short organosiloxane units chemically bonded to an unsaturated, polymerizable group provide a means of copolymerizing such organosiloxane monomers with other monomers to achieve a compatible and therefore, transparent material.
On the other hand, organosiloxane units containing only one unsaturated, polymerizable group often times will not provide a random copolymer when copolymerized with other monomers, particularly hydrophilic monomers. This situation leads to phase separation and an opaque material. In certain cases the phase separation is not detectable visually but is evidenced in the physical properties of the material. This condition can provide a material that exhibits brittle behavior and a propensity to fracture.
The physical of highly crosslinked polymer prepared from dimethylsiloxane diacrylate oligomers has been known in the art. In general, microphase separation is suppressed as the amount of dimethylsiloxane groups in the prepolymer increases. This phenomenon is attributed to the absence of long organic sequences. Overall, the work of Katz J. Polym. Sci. Chem. Ed. 16(3) 597 (1978) teaches against the copolymerization of such reactive organosiloxane monomers since organic sequences form leading to phase separation. However, in a contact lens material it has been found desirable to include one or more comonomers to provide a proper balance of physical properties.
For contact lens applications it is therefore desirable to provide a random copolymer that contains a substantial amount of compatibilized organosiloxane units. The present invention utilizes branched organopolysiloxane structures containing multiple unsaturated, polymerizable groups. These materials, when copolymerized with other monomers provide compositions which are transparent, highly oxygen-permeable and durable. The random nature of the polymerization process is enhanced by the presence of multiple, unsaturated polymerizable groups. The compatibilization of the organopolysiloxane monomers disclosed is improved through the use of highly branched organopolysiloxane moieties.